Iodide (June 24th). There is much care required in iodizing paper; we have no hesitation in saying at present the subject has not met with sufficient attention. When the iodized paper is immersed in water, it is some time before it assumes a yellow colour. This may be accelerated by often changing the water. The brightness of the colour is by no means an index of its degree of sensitiveness—on the contrary, paper of a bright yellow colour is more apt to brown than one of a pale primrose. Too bright a yellow would also indicate an insufficient soaking; and suffering the paper to remain longer than is needful not only lessens its sensitive powers, but does much damage by removing all the size.
H. N. (Kingston). Violet-coloured glass, ground on one side, may be obtained at 11d. per square foot of Messrs. Forest and Brownley, Lime Street, Liverpool. It may also be had in London, but the price charged is much higher. This glass obstructs just a sufficient degree of light, and is most agreeable to the sitter; not much advantage accrues from the use of large sheets, and it is objectionable for price. No doubt such an application as you mention would be useful; but, from the difficulty there is in keeping out the wet from a glass roof, it would be very objectionable. Beyond a reference to our advertising columns, we cannot enter upon the subject of the prices of chemicals and their purity. In making gun cotton, the time of immersion in the acids must be the same for twenty grains as for any large quantity: when good, there is a peculiar crispness in the cotton, and it is quite soluble in the ether. If our Correspondent (who expresses so much earnestness of success) will forward his address, he shall receive a small portion made according to Dr. Diamond's formulary, which we find extremely soluble; and he can compare it with that of his own production.
F. M. (Malta). 1st. We are informed by Dr. Diamond that however beautiful the results obtained by others in the use of Canson's paper, in his hands he has found no certainty in its action, and, for iodized paper for negatives, far inferior to the best English papers. If the salts of gold are to be used, deep tints are very readily obtained by the French papers. The propriety of using gold is very questionable, not only as affecting the after permanence of the picture, but from the strong contrasts generally produced being very offensive to an artist's eye. 2ndly. Xyloidine may be iodized precisely the same as collodion, but no advantage whatever is gained from its use. A collodion for the taking of positives on glass should be differently made to one for negative pictures. There should be less of the iodides contained in it, and it should be more fluid. When this is the case, the image is never washed out by the hypo., and the delineation is equal in minuteness to any Daguerreotype on metal plates, as has been shown by the specimens of the reduction of printing exhibited by Mr. Rosling at the Society of Arts' Exhibition, where the letters were reduced to 1-750th of an inch, or less than half the diameter of a human hair. If the protonitrate of iron properly prepared be used in the development, the deposit assumes the beautiful appearance of dead white silver, having none of the reflecting qualities of the metal plates.
C. E. F. (June 13th). The spots in the specimen sent depend upon minute substances in your collodion not receiving the action of the nitrate of silver bath; and you will find this upon looking through a prepared plate after it has been in the nitrate bath, and previously to its ever having been in the camera. They may be iodide or iodate of silver, or small crystals of nitrate of potash. If the former, add a little piece of iodide of potassium, say ten grains to two ounces of collodion; or if the latter, it would depend upon a defective washing of the gun cotton by which all the soluble salts have not been removed: thus more care must be used. We would recommend you to use an entirely new bath and stronger, four ounces of hypo. to a pint: it is evident that your very nice specimens have been spoiled by the stains of the bath. Allow us again to draw your attention to the process given by Mr. Pollock; we have seen most satisfactory pictures produced by it.
R. H. Chattock (Solihull). The "freckled" appearance which you mention in your positives in all probability depends upon the action of the light upon the silver, which still remains in your proof. We have often found it to be the case when old hyposulphite of soda is used, and when the strength of the bath is becoming weak and doubtful. It is certainly a safe process to soak the picture in clean water for an hour or two, the light being excluded previous to the immersion into the hypo.; and the water extracting a large portion of the solutions remaining on the paper, the after application of the hypo. need not be so long continued, whereby the tone of the picture is not so much lowered. Your own observation, that a piece of Whatman's paper being merely divided, and one point exhibiting the defects and the other not, at once negatives the idea that the size in the paper has been affected.
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The Twenty-eighth Edition.